This Security Advisory is about an object injection vulnerability in the SiteAccessMatchListener of eZ Platform, which could lead to remote code execution (RCE), a very serious threat. All sites may be affected.
Update: There are bugs introduced by this fix, particularly but not limited to compound siteaccess matchers. These have been fixed in ezsystems/ezpublish-kernel v5.4.15 and v22.214.171.124. Fixes for v7.5 and ezsystems/ezplatform-kernel v1.0 are not yet ready, they are expected on the 28th/29th.
This security update is distributed via Composer, see "Resolving versions" above.
This issue was reported to us by Serhey Dolgushev from Contextual Code. We are very grateful for their research, and responsible disclosure to us.